The question is difficult to answer quickly, and perusing one or more works on Japanese history would be the best route to providing yourself with a satisfactory answer.
The Japanese archipelago has been inhabited for tens of thousands of years, and cultural or political groupings that would eventually become the various Japans date from preliterate times.
In its present form, Japan can be dated to perhaps the late 1860s, when the final civil wars ended, but Japan expanded in one form or another greatly in the mid-20th century (ca. 1931) with the creation of a puppet (and effectively vasal) state in northeastern China; others would hold that the earlier effective annexation of Korea would consititute the beginning of expansion.
Japan became much smaller after its defeat in the Second World War and did not assume its present form until 1972, when Okinawa was returned to the administrative control of Japan.
Many Japanese would hope for a further expansion of the national territory from its present boundaries with the return of the "northern territories," siezed by the Soviet Union during the Second World War and presently controlled by Russia.
Japan Japan was created on 2007-08-08.
Seismological Society of Japan was created in 1880.
Rises Over Japan was created in 1985-03.
Japan Air Commuter was created on 1983-07-01.
Thirty-Second Army - Japan - was created in 1944.
Japan Transocean Air was created on 1967-06-20.
My Japan was created in 1945.
Pray for Japan was created in 2012.
The Japan Album was created in 1999.
Nickelodeon - Japan - was created in 1998.
The Japan That Can Say No was created in 1989.
Sketches of Japan was created in 1999.
Japan Habba was created in 2005.
Jetstar Japan was created in 2011.
Mozilla Japan was created in 2004.
The Japan Academy was created in 1879.
Telenet Japan was created in 1983.
Japan Today was created in 2000.